Methods and systems for improving optical flatness in a path length control driver

ABSTRACT

A piezoelectric transducer is described that is configured for use within a path length control apparatus of an optical device. The transducer comprises at least one void formed within a central region of the piezoelectric transducer, the one void or alternatively, the multiple voids, utilized at least in part to limit a curvature induced into a mirror during operation of the piezoelectric transducer.

PRIORITY CLAIM

This patent application claims priority from copending U.S. patentapplication Ser. No. 11/046,242 filed Jan. 28, 2005, and entitled,“Methods and Systems for Improving Optical Flatness in a Path LengthControl Driver,” now issued as U.S. Pat. No. 7,382,463, the contents ofwhich are hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

This invention relates generally to a path length control apparatus(PLC) for optical devices, and more specifically, to a PLC apparatusthat includes an improved surface for reflecting laser beams within aring laser gyroscope (RLG).

A ring laser gyroscope (RLG) is commonly used to measure the angularrotation of an object, such as an aircraft. Such a gyroscope has twocounter-rotating laser light beams that propagate within a closed loopoptical path or “ring” with the aid of successive reflections frommultiple mirrors. The closed path is defined by an optical cavity thatis interior to a gyroscope frame or “block.” In one type of RLG, theblock includes planar top and bottom surfaces that are bordered by sixplanar sides that form a hexagon-shaped perimeter. The block issometimes referred to as a laser block assembly. Three planarnon-adjacent sides of the block form the mirror mounting surfaces forthree mirrors at the corners of the optical path, which is triangular inshape.

Operationally, upon rotation of the RLG about its input axis (which isperpendicular to and at the center of the planar top and bottom surfacesof the block), the effective path length of each counter-rotating laserlight beam changes. A frequency differential is produced between thebeams that is nominally proportional to angular rotation. Thisdifferential is then optically detected and measured by signalprocessing electronics to determine the angular rotation of the vehicle.To maximize the signal out of the RLG, the path length of thecounter-rotating laser light beams within the cavity must be adjusted.Thus, RLGs typically include a path length control apparatus (PLC), thepurpose of which is to control the path length for the counter-rotatinglaser light beams to maximize the output signal.

Such PLCs typically include a piezoelectric transducer (PZT) secured toa mirror that is in turn secured to a mirror mounting surface of thelaser block assembly (LBA). The mirror is in communication with bores inan optical cavity of the LBA. The bores form a portion of the closedloop optical path or ring defined by the optical cavity. The mirrorreflects the counter-rotating laser light beams at its respective cornerof the closed loop optical path. As such, a flatness of the mirror canaffect the reflection of the counter-rotating laser light beams and thusoperation of the RLG.

SUMMARY OF THE INVENTION

In one aspect, a piezoelectric transducer configured for use within apath length control apparatus of an optical device is provided. Thetransducer comprises at least one void formed within a central region ofthe piezoelectric transducer. The single void, or alternatively themultiple voids, are utilized at least in part, to limit a curvatureinduced into a mirror during operation of the piezoelectric transducer.

In another aspect, a method for limiting an amount of curvature inducedinto a mirror during operation of a piezoelectric device attached to themirror is provided. The piezoelectric device includes one or morepiezoelectric layers adjacently stacked and the process comprisesforming a void through a central region of at least the piezoelectriclayer adjacent the mirror and attaching a non-piezoelectric stiffeningblock to the mirror within the void.

In still another aspect, a method for limiting an amount of curvatureinduced into a mirror during operation of a piezoelectric deviceattached to the mirror is provided. The piezoelectric device includesone or more piezoelectric layers and the method comprises coating asurface of at least one of the piezoelectric layers with an electrodematerial, the electrode material having a void formed therein adjacent acentral region of the respective piezoelectric layer, and forming astack of piezoelectric layers, the one or more coated surfacessubstantially parallel to a reflective surface of the mirror.

In yet another aspect, a ring laser gyroscope is provided that comprisesa laser block assembly, a mirror, and a piezoelectric transducer. Thelaser block assembly comprises an optical path bored therein and themirror comprises a reflective surface and a non-reflective surface. Thereflective surface is attached to the laser block assembly and inoptical communication with the optical path. The piezoelectrictransducer is attached to the non-reflective surface of the mirror, andcomprises at least one void located in a central region of thepiezoelectric transducer. The void is configured to limit a curvatureinduced into the mirror during operation of the piezoelectrictransducer.

In another aspect, a path length control apparatus for a ring lasergyroscope is provided that comprises a mirror comprising a reflectivesurface and a non-reflective surface and a piezoelectric transducer. Thereflective surface is configured for attachment to a laser blockassembly of the ring laser gyroscope and the piezoelectric transducer isattached to the non reflective surface of the mirror. The piezoelectrictransducer comprises at least one void located in a central region ofthe piezoelectric transducer and the at least one void is configured tolimit a curvature induced into the mirror during operation of the pathlength control apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a path length control apparatus,including a piezoelectric transducer, that is attached to a laser blockassembly.

FIG. 2 is a cross-sectional view of one embodiment of piezoelectrictransducer.

FIG. 3 is a cross-sectional view of a piezoelectric transducer whichincludes voids formed therein.

FIG. 4 is a top view of the piezoelectric transducer of FIG. 3.

FIG. 5 is a side view illustrating the operation of a non-piezoelectricblock placed within the voids of the piezoelectric transducer of FIG. 3,with respect to a mirror of the piezoelectric transducer.

FIG. 6 is a cross-sectional view of another embodiment of piezoelectrictransducer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a path length control (PLC) apparatus 10 attached toa laser block assembly (LBA) 12 of a ring laser gyroscope (RLG). PLCapparatus 10 includes a piezoelectric transducer (PZT) 16 which issecured to a mirror 18 via an epoxy-based adhesive 20. Epoxy adhesive 20covers the interface (defined by a lower surface 22 of PZT 16 and anupper surface 24 of mirror 18) between PZT 16 and mirror 18. Mirror 18is secured to a mirror mounting surface 26 of LBA 12. Mirror 18 isconfigured for communication with laser bores 32 within an opticalcavity 34 of LBA 12. Bores 32 form a portion of a closed loop opticalpath 38 defined by the optical cavity 34.

As illustrated by FIG. 1, mirror 18 reflects counter-rotating laserlight beams 40 at a respective corner of the closed loop optical path38. PZT 16 includes at least a pair of piezoelectric elements, orlayers, 42 and 44. A plurality of piezoelectric layers, for examplepiezoelectric elements 42 and 44, are sometimes collectively referred toas a piezoelectric device. PZT 16 utilizes a voltage applied topiezoelectric elements 42 and 44 and delivered by a regulated voltagesource (not shown) which is attached to contacts 46. Contacts 46 areelectrically connected to piezoelectric elements 42 and 44. Regulationof the applied voltage is in response to a signal provided by a detector(not shown) that monitors the intensity of the light beams 40.Application of the applied voltage results in small, but preciselycontrolled, mechanical movements of piezoelectric elements 42 and 44 ina direction perpendicular to a top surface 48 of PZT 16. This mechanicalmovement of piezoelectric elements 42 and 44 of PZT 16 affectstranslational movement of mirror 18, and thereby controls the pathlength of the laser light beam 40 (e.g., a length of closed loop opticalpath 38).

FIG. 2 is a cross-sectional view of a multi-layered PZT 60, whichincludes a stack 61 of alternating negative and positive co-firedceramic piezoelectric layers. Co-fired ceramic piezoelectric layers arelayers that are “fired” together when they are fabricated, as opposed tobeing fabricated separately and then later bonded together in amulti-layered stack. Multi-layered PZT 60 may include, for example, atop layer 62, a bottom layer 68, and alternating negative 64 andpositive 66 layers therebetween. Multi-layer PZT 60 also includescontacts 70, which are electrically connected to one or more of theabove described layers within multi-layer PZT 60. Such contacts aretypically formed directly on top layer 62 of PZT 60. A regulated voltagesource can be coupled directly to PZT 60 utilizing contacts 70 on toplayer 62. Multi-layer PZT 60 therefore includes a plurality of ceramiclayers including top layer 62, negative layers 64, positive layers 66,and bottom layer 68 so as to form a stack 61 in which each ceramic layerhas first and second opposing surfaces.

In one embodiment, top layer 62 includes a top conductive pattern formedon its first surface 72. The top conductive pattern includes a negativecontact 74 and a positive contact 76. Bottom layer 68 also includes aconductive pattern formed on its first surface. Layers 64 and 66 eachinclude alternating conductive patterns formed on the first surfacethereof. In such a multi-layer configuration, the co-fired ceramiclayers 62, 64, 66, 68 are more tightly coupled to mirror 18 since theylack an epoxy layer between each ceramic layer. Therefore, almost all ofthe distortion in the ceramic stack 61 is directly imparted into mirror18.

Sometimes, with conventional PZTs, for example, PZT 16 and multi-layeredPZT 60, in which the PLC driver is bonded directly to the transducermirror, curvature in the mirror due to stresses or other factors maycause multi-moding of the laser beam that is directed towards (andreflected from) mirror 18. In multi-layered PZT 60, this multi-modingoccurs more often, for example, in approximately 30-50% of the laserblock assemblies which utilize a PZT similar to PZT 60. This isparticularly true, for example, because only thin layers 20, forexample, from about 0.0005″ to about 0.001″ of epoxy are typically usedto attach the mirror 18 to the driver. This multi-moding interferes withthe laser mode that the LBA 12 uses to get accurate count data (andtherefore navigation data).

FIG. 3 is a cross-sectional view multi-layered PZT 100, which includes astack 102 of alternating negative and positive co-fired ceramicpiezoelectric layers, 104 and 106 respectively, attached to mirror 108with a layer of epoxy 110. Contacts 112 attached to top layer 114operate similarly to contacts 70 (shown in FIG. 2) as described above.Although not shown in FIG. 3, piezoelectric layers 104 and 106 includehaving alternating conductive patterns formed on their top surfacesproviding a mechanism for electrical contact with contacts 112.

While generally similar to PZT 60 (shown in FIG. 2 and described above),ceramic layers 104 and 106 of PZT 100 each include a void 120 or holeformed therethrough. Void 120 is roughly centered at a positiongenerally co-linear to a perpendicular of the mirror and within an area122 where a laser beam strikes mirror 108. In one embodiment, void 120is circular in shape and therefore, ceramic layers 104 and 106 have ashape similar to a washer. In other embodiments (not shown), the void isconfigured in other geometric shapes, including, but not limited to, asquare, a rectangle, and an oval. Area 122 is sometimes referred to as acritical region. Within void 120 is a block 124 of non-piezoelectricmaterial that is also bonded to mirror 108 with epoxy 110. By bondingblock 124 to mirror 108, area 122 of mirror 108 is constrained to retaina surface with an improved flatness as compared to mirrors within knownPZTs, for example PZTs 16 and 60.

The bonded non-piezoelectric material (e.g., block 124) acts through thebond of epoxy 110 to distribute stresses placed on mirror 108 by changesin temperature and voltage, for example, from mirror to block 124. Asurface area of block 124, in one embodiment, is greater than area 122since stress concentrations are greatest at a perimeter of block 124.Therefore, moving area 122 (the critical region of mirror 108) fartherfrom the perimeter of block 124 (closer to a center of block 124)reduces the effects of the stress concentrations. FIG. 4 is a top viewof PZT 100 further illustrating void 120 through top layer 114,non-piezoelectric block 124 and area 122 of mirror 108.

FIG. 5 is a side view of mirror 108, epoxy 110, and block 124 whichillustrates the stress distribution of block 124 and the improvedflatness of mirror 108 within area 122. The actual dimensions of block124 and void 120 are dependent on the amount of voltage available andthe flexibility (ease of driving the mirror) of mirror 108. However, itis important is that void 120 is of a large enough diameter so thatstresses are minimized in area 122 of mirror 108 at which the laser beam40 reflects and thus minimizes any curvature.

FIG. 6 is a side view of another embodiment of PZT 150 which is alsoconfigured to minimize a flexibility of mirror 152 within an area 154from which a laser beam reflects. More specifically, PZT 150 includes astack 156 of alternating negative and positive co-fired ceramic layers,158 and 160 respectively, attached to mirror 152 with a layer of epoxy162. On a surface of a number of layers 158 and 160, an electrodematerial 166 is screen printed thereon. Electrode material 166 is screenprinted, in one embodiment, to include a void 170 therein which islarger than a critical region 172 of mirror 152. Voids 170 renderunscreened portions 174 of layers 158 and 160 inactive. The inactiveportions 174 of layers 158 and 160 do not react to electrical signalsapplied to electrode material 166 from contacts 176 and therefore act toconstrain a flatness of mirror 152 within critical region 172.

As above, the actual dimensions of voids 170 within electrode material166 are dependent on the amount of voltage available and the flexibility(ease of driving the mirror) of mirror 152. However, and similarly tovoids 120 and non-piezoelectric block 124 in PZT 100, it is important isthat voids 170 are of a large enough diameter so that stresses areminimized in critical region 172 of mirror 152 at which the laser beam40 reflects to minimize any curvature

The above described embodiments make the path length control (PLC)mirrors for laser devices such as ring laser gyroscopes less susceptibleto beam area curvature within the mirrors due to thermal, voltage, andother displacement effects. In one embodiment, a stiffening block 124 isprovided behind a critical area 122 of mirror 108 which limits thecurvature induced into the mirror by the PLC driver. In this embodiment,a separate material (e.g., stiffening block 124) is located within voids120 formed in the piezoelectric material, 104 and 106, attached to aback side of mirror 108. In another embodiment, stiffening is providedthrough a lack of screen printed electrode material 166 (e.g., voids170) applied to piezoelectric material, 158 and 160. Voids 170 withinscreen printed material 166 is sometimes referred to as an inactive areawithin the PLC driver. The inactive area 154 in the ceramic of thepiezoelectric driver causes a decoupling of any bending motion within anactive area of piezoelectric layers 158 and 160 (e.g., the area that iscoated electrode material 166) from the inactive area 154 behind mirror152.

While PZT 100 and PZT 150 are described as being formed from co-firedceramic layers, it is to be understood that PZTs which are formed fromindividual ceramic layers that are bonded together after fabrication,for example, similar to PZT 16 (shown in FIG. 1) may benefit fromincorporation of the embodiments described herein. For example, voidsmay be formed in the ceramic layers and the epoxy utilized to bond thelayers together to facilitate insertion of a non-piezoelectricstiffening block within the void and attached to the mirror. Similarly,voids may be formed in the electrode material that is applied to thesurface of such ceramic layers, to provide a similar effect on theflexibility of a mirror to which such piezoelectric devices areattached.

While the invention has been described in terms of various specificembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theclaims.

1. A method for limiting an amount of curvature induced into a mirrorduring operation of a piezoelectric device attached to the mirror, thepiezoelectric device including one or more piezoelectric layers, saidmethod comprising: coating a surface of at least one of thepiezoelectric layers with an electrode material, the electrode materialhaving a void formed therein adjacent a central region of the respectivepiezoelectric layer; and forming a stack of piezoelectric layers, theone or more coated surfaces substantially parallel to a reflectivesurface of the mirror.
 2. The method of claim 1 wherein the void has aperimeter larger in diameter than a diameter of an area of the mirrorwhich may be impinged by a beam.
 3. The method of claim 1, wherein thepiezoelectric layers have voids corresponding to the electrode materialvoids, and further comprising: bonding a non-piezoelectric stiffeningblock to the mirror, the non-piezoelectric stiffening block in the voidsand above an area of the mirror which may be impinged by a beam.
 4. Themethod of claim 3, wherein the non-piezoelectric stiffening block isbonded to a non-reflective surface of the mirror.
 5. The method of claim1, wherein forming the piezoelectric layers are configured to notinclude a void corresponding to the void of the electrode material.
 6. Apiezoelectric transducer configured to limit an amount of induced mirrorcurvature during operation of a piezoelectric device bonded to themirror defined by a reflective surface and a non-reflective surface, thepiezoelectric transducer comprising: a plurality of stackedpiezoelectric devices substantially parallel to the reflective surfaceof the mirror, with least one piezoelectric device attached to aperiphery region of the mirror and formed with a void within a centralregion of the piezoelectric device; an electrode material coating asurface of the at least one piezoelectric device, the electrode materialhaving a void formed therein adjacent the central region of the at leastone piezoelectric device; and a non-piezoelectric stiffening blockresiding in the void with an end bonded to a central region of thenon-reflective surface of the mirror, wherein the non-piezoelectricstiffening block is operable to limit a curvature of the central regionof the mirror during operation of the piezoelectric transducer.
 7. Thepiezoelectric transducer of claim 6, wherein an opposing surface of theat least one piezoelectric device with the void is bonded to theperiphery region of the mirror.